The invention relates to a method for charging capacitors of a direct-voltage intermediate circuit of a frequency converter.
The invention also relates to an arrangement for charging capacitors of a direct-voltage intermediate circuit of a frequency converter, the arrangement comprising voltage connections to a voltage supply unit of the direct-voltage intermediate circuit, a positive bar and a negative bar of the direct-voltage intermediate circuit, a charging circuit of the direct-voltage intermediate circuit, a switch of the charging circuit and a by-pass switch of the charging circuit, capacitors corresponding to inverter parts connectable to the direct-voltage intermediate circuit, the first pole of the capacitors being connected to a positive bar of the direct-voltage intermediate circuit and the second pole being connectable in connection with a negative bar of the direct-voltage intermediate circuit by means of a charge switch and/or a main switch of the capacitor.
A frequency converter is a device, which is used for generating an adjustable alternating voltage by using a constant-frequency voltage. The most typical application of frequency converters is to control motors in such a manner that alternating voltage of a supplying network is converted into adjustable voltage in order to control the motor. In direct frequency converters, the electricity to be supplied from an alternating current network is chopped by semiconductor switches directly into an alternating current with a desired frequency and voltage. In frequency converters with intermediate circuits, both direct-voltage and direct-current intermediate circuits, the electricity supplied by the alternating current network is first converted into direct current and then back into alternating current.
A frequency converter with a direct-voltage intermediate circuit comprises a rectifier part for rectifying alternating current into direct current, a direct-voltage intermediate circuit and an inverter part for converting direct current back into alternating current. In a direct-voltage intermediate circuit, high-energy capacitors or capacitor batteries are used both as low-impedance energy reserves and for filtering the direct voltage and direct current and thus for smoothing the direct-voltage ripple. There is a separate capacitor or capacitor battery for each inverter part in the intermediate circuit. In addition to the capacitor, the direct-voltage intermediate circuit may also comprise a smoothing inductor between the rectifier part and the capacitor.
When a dead high-capacitance capacitor is connected via a low impedance to a voltage source serving as a supply unit for the direct-voltage intermediate circuit, a power surge occurs, which may cause that protective devices, such as fuses, start to function. To prevent this, the power surge must be limited to a level suitable for protective devices. If a fully-controlled or semi-controlled diode bridge serves as a supply unit for the direct-voltage intermediate circuit, the direct voltage of the intermediate circuit may be increased during the starting in a ramp-like manner by changing the current delay angle of thyristors belonging to the supply unit. If the supply unit only consists of diodes, a diode bridge or a unit which is started at first as a pure diode bridge, e.g. a network inverter, a specific charging circuit must be used in the supply unit for limiting the charging current. A charging circuit limiting the charging current may include, for instance, resistors or a direct-current chopper. After the charging, the charging circuit is typically bypassed, since too great power losses would be generated in the charging circuit if power was supplied through it in a normal operating situation.
In big factories, inverter parts are typically connected to line drives, whereby one line drive may comprise dozens of inverter parts connected to the same DC busbar system. Some of these inverter parts may be connected to the DC busbar system during the starting and some not, e.g. because of maintenance of a motor connected to the inverter part. In this case, however, it must be possible to connect each inverter part to the live DC busbar system without disturbing the other inverter parts.
Conventionally this is implemented by a plurality of separate charging circuits, whereby during the starting of the supply unit, a separate charging circuit reserved for the supply unit and a separate charging circuit for each inverter part are used. Each inverter part thus requires its own charging circuit. A problem with this solution is that in large line drives, wherein there is a separate charging circuit for each inverter part and a direct-current chopper serves as a charging circuit, the costs of the system rise considerably because of a great number of direct-current choppers. Instead of direct-current choppers, resistors may be used as components limiting the current of the charging circuit but, in this case, the sizing of the resistors, both in each individual charging circuit and in the entire system, is very difficult.